An operational amplifier in the form of a semiconductor integrated circuit generally consists of an input stage, a biasing circuit, and an output stage. The input stage is usually a differential amplifier circuit.
For example, consider a conventional operational amplifier in which the input portion of the differential-amplifier input stage consists of a pair of NPN input transistors whose bases receive the differential input signal and whose emitters are coupled together and through a current source to a low voltage supply. The collectors which provide an amplified differential signal are coupled through respective current sources to a high voltage supply. P. Gray et al illustrate such an operational amplifier in "Recent Advances in Monolithic Operational Amplifier Design," IEEE Trans. Circs. and Systs., vol. CAS-21, May 1974, pp. 317-327.